The invention relates to a method and an arrangement for purifying the gases that are to be fed to a fuel cell for operation by removing constituents which are unfavorable to the fuel cell operation.
On account of their high efficiency and the low levels or absence of pollutant emissions, fuel cells are also used in electric vehicles. By way of example, an electric vehicle is known which has a driving motor, a fuel cell and a fuel tank, a water store, an evaporator and a reformer. The fuel tank contains methanol which, together with water from the water store, is converted into the gaseous state in the evaporator and is then passed to the reformer, in which substantially hydrogen, carbon dioxide and carbon monoxide are formed with heat being supplied by a catalytic burner. The carbon monoxide can be oxidized with an oxidizing agent. The hydrogen-containing fuel gas from the reformer is fed to the fuel cell by way of a compressor; the fuel cell comprises a fuel cell stack in which a multiplicity of individual fuel cell modules are integrated. Humidified air is fed to the fuel cell by a further compressor. Electrical energy for the electric driving motor is generated in the fuel cell from the hydrogen and from the oxygen of the air (DE 44 12 450 A1).
Membrane fuel cells, which in each case have a proton-conducting ion exchange membrane made from a polymer material, e.g. fluorinated resin, with a very good electrical conductivity in the moist state, are also used in electric vehicles. The membrane surface is covered with a catalyst. On one side, the electrolyte membrane is connected to a gas-permeable anode, and on the other side, it is connected to a gas-permeable cathode. A ribbed, gas-impermeable plate adjoins the anode; the cavities between the ribs of this plate serve to supply the oxidizing gas, e.g. air, with an oxygen content. A gas-impermeable ribbed plate likewise adjoins the cathode, and its cavities between the ribs are used to supply the gaseous fuel, e.g. the hydrogen-containing gas. When the fuel cell is operating, the electrolyte membrane is moistened by the water of reaction and the humidity in the gases supplied.
A fuel cell in a fuel cell system requires sufficiently purified gases and/or gas mixtures, both with regard to the fuel gas and with regard to the oxidizing gas, for it to operate if the original gas contains constituents which have an adverse effect on the mode of operation of the fuel cell. In this context, the term fuel cell is to be understood as meaning both an individual fuel cell module with the structure as described above and a stack of fuel cell modules of this type, which can be connected in parallel and/or in series. If the oxidizing gas used is air, which is drawn in from the atmosphere, it is generally necessary to carry out a purification step. When using hydrogen, which is taken from a tank or is generated from a liquid fuel, it is often the case that there are no disruptive constituents in the gas, and consequently there is no need for any purification.
WO 02/22234 A2, which forms the generic document, discloses an air filter system for low-temperature catalytic processes for fuel cells. In this system, a very wide range of particles and gases/vapors are filtered out of the incoming air. The filters can be divided into physical or particulate filters and chemical filters. The filters are connected in series in a housing. If the concentration of a pollutant in the incoming air drops below a certain concentration, the chemical filter is spontaneously regenerated through desorption.
The invention is based on the problem of providing a method and an arrangement for eliminating contaminating substances from the gases for operation of a fuel cell before the gases are fed into the fuel cell.
In a method of the type described above, the object is achieved, according to the invention, by the fact that: (a) the gas(es) are passed across a filter system which is designed both to separate off particulates and to remove constituents in gas and vapor form which have a damaging effect on operation of the fuel cells, and (b) the gas(es) are fed to the fuel cell on leaving the filter system. In this case, the gas(es) are passed across a filter system which can be regenerated and is monitored on the basis of criteria indicating a drop in the filter action and that the regeneration should be carried out, with a message being generated when these criteria are reached. The regeneration is carried out when the fuel cell is inoperative. This makes it possible to avoid damage to the fuel cell in the event of spontaneous releases of pollutants during regeneration.
The method according to the invention removes not only particulates, such as dust and carbon particles, but also further constituents of the gas, such as polluting gases, aerosols, organic substances, such as algae, spores, bacteria and viruses, from the gas or gases, which are also referred to below as reaction gases, although they may comprise a mixture of gases, not all of which react in the fuel cell or contribute to the generation of electric power. The purification of the gas or gases makes it possible to lengthen the operating time or service life of the fuel cell.
In an arrangement of the type described above, the object is achieved, according to the invention, by virtue of the fact that a filter system is arranged at a location in the gas-carrying passage for feeding the gas to be purified to the fuel cell, which filter system separates out both particulates and constituents in gas or vapor form, which have a damaging effect on operation of the fuel cell. In this case, the gas(es) are passed across a filter system which can be regenerated and can be monitored on the basis of criteria indicating a drop in the filter action and the execution of the regeneration; a message can be generated when these criteria are reached. The purification of the respective reaction gas using the filter system prevents impurities from being deposited in the feed passages, in delivery devices and in the fuel cell itself, thereby gradually causing the function of the fuel cell to deteriorate, or prevents polluting gases from causing undesirable reactions in the fuel cell.
In an expedient embodiment, the filter system has a first filter for particulates, downstream of which there is a second filter with a substance for taking up and binding pollutants in gas or vapor form. Dry filters made from plastic, glass fiber, paper with a high level of dedusting can be used as the first filter and may, for example, have a labyrinth-like structure. The second filter includes, in particular, porous substances for taking up and physically or chemically binding gases or vapors at the surface. Examples of substances of this type include activated carbon or kieselguhr.
In another expedient embodiment, the filter system comprises a unit in which a dry filter for particulates and a substance for taking up and binding gases or vapors at its surface are arranged together. Therefore, the filter system combines the functions of particulate separation and removal of polluting gases. It is expedient for the particulate filter to include a substance for binding and/or separating off gases, which substance is arranged on a material for separating out particulates or is self-supporting or forms a bulk bed.
In one preferred embodiment, the filter system is designed such that it can be regenerated, it being possible for the regeneration to be triggered by an actuating element. This makes it possible to prevent the regeneration from being triggered spontaneously, e.g. when the fuel cell is operating, which can lead to high levels of pollutants being emitted and therefore to damage to the fuel cell. The regeneration of the first filter can be carried out, for example, using compressed air, whereas the regeneration of the second filter can be effected by increasing the temperature, since the adsorption is lower at a higher temperature than at a lower temperature.
It is advantageous if the filter system is arranged in the gas-carrying passage for the oxidizing gas upstream of the gas inlet of a compressor. The gas-carrying passage may be arranged inside or outside the fuel cell system to which the fuel cell belongs.
To establish the need for regeneration or filter maintenance with regard to particulate separation, in particular, the pressure difference between the pressure upstream and downstream of the filter system is compared with a predeterminable limit value, with a message being generated if the latter is exceeded.
With regard to the adsorption of pollutants, the need to regenerate or carry out maintenance on the filter can be established using one or more gas or pollutant sensors downstream of the filter system, which are set to measure the pollutants which are to be separated off. The measured values from the sensors are compared with a limit value in each case, with a message being generated if the latter is exceeded.